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E-tattoos turn knuckles and freckles into smartphone controls

Smartphone controller

By Matt Reynolds

Make the most of that beauty spot. Ultrathin temporary electronic tattoos can now turn body blemishes into touch-sensitive buttons, letting you control your smartphone with your own wrinkles, freckles and other skin features.

People intuitively know the location of their own bumps and birthmarks, which makes them ideal locations for touch-sensitive buttons, says Martin Weigel at Saarland University in Saarbrücken, Germany, who has led the research. You could squeeze a freckle to answer a phone call, or slide a finger over your knuckles to change the volume of your music.

Weigel and his colleagues at Saarland University and Google used conductive ink to print wires and electrodes on temporary tattoo paper. The tattoos, which they call SkinMarks, are thinner than the width of a human hair. They are transferred onto the skin using water and last a couple of days before rubbing off.

“We make use of the elastic properties of the skin, including bending and stretching,” says team member Jürgen Steimle, also at Saarland University. By having the tattoos responsive to changes in the skin surface, they incorporate multiple commands at one location.

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Pump up the volume

For example, you could adjust your smartphone volume by sliding one finger across a tattoo placed along the side of another finger. But bend the tattooed finger and the volume slider could become a play and pause button. Similarly, tattoos on the knuckles could act as four distinct buttons when the hand is making a fist, but then function as one long slider when the fingers are extended.

Another of the touch-sensitive tattoos is electroluminescent, glowing when a current passes through it. You could have tattoos in the shape of icons representing your favourite smartphone apps, which would light up when you receive a notification, says Weigel.

The team has also placed a heart-shaped tattoo over a volunteer’s birthmark that can glow when a nominated loved one is available for a phone call. Touching the tattoo would call that person’s phone. The group will present the work at a computer-human interaction conference in Denver, Colorado in May.

To test the tattoos, the group connected them to a computer, but in future they will link them to Android smartphones. “Now we’ve tested the technological feasibility, the next step is to look at implementing it in a practical way,” says Weigel.

The new normal

One practical hurdle is shrinking the microcontrollers used to transmit signals from the tattoos to a computer or smartphone. For this study, Weigel’s team used copper tape to connect the tattoos to a small Arduino microcontroller attached to the body with a wristband, but the bulky circuit boards would be uncomfortable to wear on other parts of the body.

“This is amazing research,” says Chris Harrison at Carnegie Mellon University. People have experimented with electronic tattoos before, but by basing the device on cheap temporary tattoos, Weigel’s team has made technology that could be used by artists, programmers and hobbyists, he says.

Despite the current limitations, Harrison says on-skin devices are the next logical step in wearable technology. “Human fingers are quite nimble on their own skin,” he says. And a hand provides a greater surface area than current smartwatch screens.

It will be 10 years before we see touch-sensitive tattoos in mainstream use, says Harrison, but he predicts a future in which skin-based controls are the new normal. “You’ll have these digital tattoo parlours which you can go to in 2050 and 5 minutes later you can walk out with the iPhone 22 on your forearm.”